(a) Field of the Invention
The present invention relates to fuel cell systems and driving methods thereof. More particularly, the present invention relates to a fuel cell system for reducing a cathode catalyst, and easily removing reactants and non-reactants created inside the fuel cell.
(b) Description of the Related Art
A fuel cell is a device that electrochemically generates electric power using fuel (e.g., hydrogen or reformed gas) and an oxidizer (e.g., oxygen or air). That is, the fuel cell constantly receives fuel (e.g., hydrogen or reformed gas) and an oxidizer (e.g., oxygen or air) from an external device, and transforms the fuel and oxidizer directly to electrical energy through an electrochemical reaction.
Pure oxygen, or oxygen-rich air (i.e., air containing a large amount of oxygen), is used as the oxidizer of the fuel cell, and pure hydrogen, or a hydrogen-rich fuel (i.e., a fuel containing a large amount of hydrogen), is used as the fuel. The hydrogen-rich fuel may be a fuel produced by reforming a carbonized hydrogen fuel, such as liquefied natural gas (LNG), liquefied petroleum gas (LPG), and CH3OH.
A polymer electrolyte membrane fuel cell (PEMFC) is one example of a fuel cell system. The PEMFC has high output density and high energy transformation efficiency, and is operable at low temperatures, for example below 80° C. Also, the PEMFC is down-sizable, closeable, and sealable. Therefore, the PEMFC can generally be used as a power source in many fields, for example electromobiles, home power generators, mobile communication equipment, military equipment, and medical equipment.
A PEMFC includes a reformer for generating a reformed gas containing a large amount of hydrogen from a fuel, and a fuel cell stack for generating electric power from the reformed gas. The fuel cell stack receives oxygen and the reformed gas and generates electric power through the reaction of the oxygen and the hydrogen.
The PEMFC is driven at a high driving voltage, enabling reduction in the use of fuel, resulting in high driving efficiency. However, the high voltage driving of PEMFC causes the cathode in the fuel cell stack to be oxidized, thereby deteriorating cathode catalyst activation. Carbon-supported platinum catalysts (i.e., platinum particles supported in carbon particles) are generally used as the cathode catalysts in the cathodes of fuel cell stacks. When an oxidizer is supplied to the fuel cell stack, an oxide film is formed on the cathode catalyst. This oxide film deteriorates catalyst activation, and the oxidizer causes the carbon particles to oxidize while driving.
Also, non-reactants are generated during reaction. If the non-reactants are not removed, the non-reactants block channels of the stack. As a result, flooding problems may arise. Flooding problems are the major cause of decreases in the life-spans of fuel cell stacks.
The above information disclosed in this Background section is presented to enhance understanding of the invention, and therefore may contain information that does not form part of the prior art known to those of ordinary skill in the art.